1. Field of the Invention
The present invention relates to a method and an apparatus for measuring the damping characteristics of a friction member with respect to vibration.
2. Description of the Related Art
In a conventional frictional vibration evaluation device (such as those described in Japanese Patent Application Laid-open Nos. 62-832 and 5-126683), a vibration system analogous to equipment to be practically used is disposed on a testing machine, and vibrations which are considered to actually arise in the equipment are simulated by the testing machine. Vibrations which would arise in actual equipment are predicted on the basis of the presence or absence of vibrations in the testing machine.
More specifically, in a testing machine such as that shown in FIG. 6, a brake shoe B, which serves as a friction member F made of friction materials, is attached to a main body H joined to a rotary shaft R which is rotated by a motor M, and the diameter of a torsion shaft T fixed to the main body H is reduced in order to facilitate the occurrence of judder. A torque meter TM is coupled to the torsion shaft T. Through use of the foregoing testing machine, a judder test is carried out in a test room without a car being subjected to test run.
The conventional testing machine allows only an evaluation as to whether or not the friction member F which is to be evaluated has the capability to cause frictional vibrations of a certain magnitude or higher. More specifically, the conventional testing machine determines only whether or not the friction member F causes frictional vibrations of such a magnitude as to exceed the inherent damping capability of the equipment.
The conventional frictional vibration evaluation device merely classifies friction members to be evaluated into two types, that is, a material which causes vibrations and a material which does not cause any vibrations, wherein only the damping capability of the evaluation device is used as a criteria for classification. The conventional frictional vibration evaluation device cannot evaluate the friction member in further detail.
To a certain extent, the conventional frictional vibration evaluation device can be used to evaluate the superiority or inferiority of the material on the basis of the magnitude of vibrations produced. However, the evaluation device can find only that a certain friction member causes frictional vibrations in the evaluation apparatus, i.e., the friction member has inferior vibration-suppressing capability, but cannot be used as an evaluation apparatus for friction members having superior damping characteristics.
In practice, variations exist in the rotational speed of the rotary shaft, and these variations cause vibrations that continue for a long period of time. The period of the continuation depends on the relationship between the magnitude of damping or vibration-suppressing characteristics and that of excitation characteristics of an equipment and a friction member. Various variations exist in a machine to be actually used, and hence vibrations are apt to arise. Vibrations such as judder are phenomena in which vibrations occur at the eigenfrequency (natural frequency) of the equipment and continue for a long period of time. The vibrations occurring at the eigenfrequency are characterized by the phenomenon that the greater the damping in the vibration system, the smaller the vibration amplitude caused by vibration force imparted to the vibration system, thereby preventing the vibrations from becoming greater. In order to prevent vibrations from continuing for a long period of time in every situation, the friction member itself must have no excitation characteristics and must have high resistance, or high damping performance against external vibrations. The aforementioned conventional frictional vibration evaluation device performs an evaluation only as to whether or not the friction member causes vibrations, but is incapable of evaluating the friction member with regard to given vibrations which are important factors in practical use.